Integrated circuit (IC) products can be found in most electronic devices throughout the world. The most familiar applications of IC products are found in cellphones, camcorders, portable music players, televisions, and computers. Almost all of these product types employ some form of memory, whether it be read-only memory (ROM), or random-access memory (RAM). ROM and RAM can also be classified as nonvolatile and volatile memory, respectively. Volatile memory is a semiconductor device that allows data to be stored and changed as desired; data in the volatile memory is lost when the power is turned off. Nonvolatile memory is a semiconductor device designed to store digital data in the form of an electrical charge, wherein the charge remains in storage even after the power is turned off.
Flash memory is a type of nonvolatile memory that can be erased and reprogrammed many times. Flash memory is usually constructed by one of two technologies, floating gate or silicon-oxide-nitride-oxide-silicon (SONOS). Recently, SONOS technology is considered to be a promising solution for not only nonvolatile stand alone memory, but also embedded memory due to its numerous advantages, such as, excellent scalability, low power consumption, process simplicity, and logic process compatibility.
Unfortunately, flash memory, including SONOS flash, is usually required to force erase bias for 64 kbytes simultaneously (called a “sector erase”). This sector erase method can suffer from low erase quality unless every bit within the sector displays uniform characteristics. The properties affecting uniform characteristics between bits are: active region critical dimension, channel mobility stresses due to shallow trench isolation (STI) formations, and charge trapping in oxide-nitride-oxide (ONO) layers of the SONOS. The problems associated with these properties are exacerbated by the large sector sizes used in today's flash memories.
Large sector sizes require long wordlines and source lines, which are prone to resistive/capacitive (RC) delay due to their material composition. For instance, the RC delay of a polysilicon wordline exceeds that of a metal bitline. Consequently, strapping of the wordlines with metal lines is required in order to achieve reasonable access times. Strapping is the process of shorting a polysilicon wordline, by electrically interconnecting the wordline to a metal line, to significantly reduce the resistance and access time of the wordline.
Strapping requires the use of drop contacts within contact regions, every certain distance. Drop contacts are electrical interconnections formed between the polysilicon wordlines and the metal lines. Unfortunately, these drop contacts require extra area that create discontinuities in layout pattern spacing. These discontinuities in spacing detrimentally affect the performance of the bits adjacent the drop contacts.
The bits adjacent the drop contact regions have a tendency to display dissimilar characteristics from other bits. These dissimilar characteristics arise due to two kinds of problems. The first is that the active critical dimension (CD) region of the bits nearest the drop contact regions are different from that of other bits, and the mobility of the channel may be different due to STI stresses as well. The other problem is that the nearest bit usually suffers from plasma damage incurred during the contact and via etch processes. During the etch of the drop contact regions, charged particles from the plasma disperse into the polysilicon gate line and the ONO layer, where most of the charges can be trapped in the nitride film of the closest bits, and degrade ONO integrity.
Thus, a need still remains for a memory array with uniform cell characteristics that increases the reliability of sector erasures. In view of the ever-increasing need to save costs and improve efficiencies, it is more and more critical that answers be found to these problems.
Solutions to these problems have been long sought but prior developments have not taught or suggested any solutions and, thus, solutions to these problems have long eluded those skilled in the art.